864 



SCIENCE. 



[N. S. Vol. II. No. 52- 



tangle a law of nature from the phenomena 

 which men always had before their eyes, 

 but whose explanation had nevertheless 

 always escaped the inquiry of the philoso- 

 pher." 



By this discovery Galileo laid the foun- 

 dation of the modern science of Dynamics. 

 ]S"ewton completed the work so well begun ; 

 the result is summed up in the three Axioms 

 or Laws of Motion. 



Newton's discovery of the law of univer- 

 sal gravitation furnished a splendid illus- 

 tration of the dynamical concept of force. 

 Newton also supplemented his physical dis- 

 coveries by a mathematical discovery of 

 equal magnitude. I mean the discovery of 

 the infinitesimal calculus. It enabled him 

 to express the new idea of force by a mathe- 

 matical formula of rare simplicity, for noth- 

 ing could be simpler than the formula which 

 states that the flux or rate of change of 

 momentum equals the moving force. This 

 simplicity gave to the dynamical idea of 

 force a fresh and almost irresistible charm. 

 The Newtonian force considered formally is 

 a mathematical symbol, a rate of variation, 

 or, as the mathematician calls it, a differ- 

 ential coefficient, and this symbol con- 

 sidered physically conveys to our mind 

 nothing more than simply a description of 

 the instantaneous state of motion. It mat- 

 ters little what the mechanism is by means 

 of which the moving body receives the im- 

 pulses of the force. Newton's Dynamics 

 did not explain the mechanism by means 

 of which material bodies gravitate toward 

 each other, nor did it suggest anj^ immediate 

 need for such an explanation. The liyj)Othe- 

 sis of direct action at a distance worked 

 just as well as any other hypothesis, and 

 it had the advantage of settling aimless 

 discussions quickly when the Science of 

 Dynamics was too busy with numerous 

 important problems awaiting solution to 

 waste its time on needless speculation ; 

 and, besides, there were really none but 



purely metaphysical arguments that could 

 be brought in the case of gravitational force 

 against this hj'pothesis of direct action at a 

 distance. 



But, unfortunately, that which at one 

 time was looked upon as a convenient 

 hypothesis threatened to become a fixed 

 scientific creed. Newton's dynamics con- 

 sidered force in its aspect of a law of motion 

 expressible by a simple mathematical sj'm- 

 bol and nothing else ; but just as the Greek 

 mind saw an active divinity in every phys- 

 ical phenomenon, and Thales ascribed the 

 electrical attraction of amber to the mani- 

 festation of a universal soul, and Gilbert 

 perceived the activity of an occult virtue 

 in a magnet, so the so-called Newtonian 

 school ascribed an objectively active exist- 

 ence to the law of gravitation and called it 

 the force of gravitation. Even more than 

 that. To Newton the force of gravitation was 

 something merely descriptive ; to the New- 

 tonian school of mathematical physicists it 

 was an attribute of matter that had an ob- 

 jectively active existence in consequence of 

 which matter could act upon matter directly 

 at a distance. " It is true," says Maxwell,* 

 " that at one time those who speculated as to 

 the causes of physical phenomena were in the 

 habit of accounting for each kind of action 

 at a distance by means of a special setherial 

 fluid, whose function and property it was 

 to produce these actions. They filled all 

 space three and four times over with ethers 

 of different kinds, the properties of which 

 were invented merely to save appearances, 

 so that more rational enquirers were willing 

 rather to accept not only Newton's definite 

 law of attraction at a distance, but even the 

 dogma of Cotes, that action at a distance is 

 one of the primarj^ properties of matter, and 

 that no explanation can be more iutelligible 

 than this fact. Hence the undulatory theoiy 

 of light has met with much opposition direc- 

 ted not against its failure to explain phe- 



* Treatise on Eleo. and Mag., 2d ed., p. 448. 



